Analysis of relative effectiveness of high-dose versus standard-dose influenza vaccines using an instrumental variable method

Application of methodological strategies to address unmeasured confounding in real-world vaccine safety and effectiveness study: a systematic review

OBJECTIVES

Uses of real-world data to evaluate vaccine safety and effectiveness are often challenged by unmeasured confounding. The study aimed to review the application of methods to address unmeasured confounding in observational vaccine safety and effectiveness research.

STUDY DESIGN AND SETTING

We conducted a systematic review (PROSPERO: CRD42024519882), and searched PubMed, Web of Science, Embase, and Scopus for epidemiological studies investigating influenza and COVID-19 vaccines as exposures, and respiratory and cardiovascular diseases as outcomes, published between January 1, 2017, and December 31, 2023. Data on study design and statistical analyses were extracted from eligible articles.

RESULTS

A total of 913 studies were included, of which 42 (4.6%, 42/913) accounted for unmeasured confounding through statistical correction (31.0%, 13/42) or confounding detection or quantification (78.6%, 33/42). Negative control was employed in 24 (57.1%, 24/42) studies-2 (8.3%, 2/24) for confounding correction and 22 (91.7%, 22/24) for confounding detection or quantification-followed by E-value (31.0%, 13/42), prior event rate ratio (11.9%, 5/42), regression discontinuity design (7.1%, 3/42), instrumental variable (4.8%, 2/42), and difference-in-differences (2.4%, 1/42). A total of 871 (95.4%, 871/913) studies did not address unmeasured confounding, but 38.9% (355/913) reported it as study limitation.

CONCLUSION

Unmeasured confounding in real-world vaccine safety and effectiveness studies remains underexplored. Current research primarily employed confounding detection or quantification, notably negative control and E-value, which did not yield adjusted effect estimates. While some studies used correction methods like instrumental variable, regression discontinuity design, and negative control, challenges arise from the stringent assumptions. Future efforts should prioritize developing valid methodologies to mitigate unmeasured confounding.

Protection against influenza hospitalizations from enhanced influenza vaccines among older adults: A systematic review and network meta-analysis

BACKGROUND

Influenza vaccines are available to help protect persons aged ≥65 years, who experience thousands of influenza hospitalizations annually. Because some influenza vaccines may work better than others, we sought to assess benefit of high-dose (HD), adjuvanted (ADJ), and recombinant (RIV) influenza vaccines ("enhanced influenza vaccines") compared with standard-dose unadjuvanted influenza vaccines (SD) and with one another for prevention of influenza-associated hospitalizations among persons aged ≥65 years.

METHODS

We searched MEDLINE, Embase, CINAHL, Scopus, and Cochrane Library to identify randomized or observational studies published between January 1990 and October 2023 and reporting relative vaccine effectiveness (rVE) of HD, ADJ, or RIV for prevention of influenza-associated hospitalizations among adults aged ≥65 years. We extracted study data, assessed risk of bias, and conducted random-effects network meta-analysis and meta-regression.

RESULTS

We identified 32 studies with 90 rVE estimates from five randomized and 27 observational studies (71,459,918 vaccinated participants). rVE estimates varied across studies and influenza seasons. Pooled rVE from randomized studies was 20% (95% CI -54 to 59) and 25% (95% CI -19 to 53) for ADJ and HD compared with SD, respectively; rVE was 6% (95% CI -109 to 58) for HD compared with ADJ; these differences were not statistically significant. In observational studies, ADJ, HD, and RIV conferred modestly increased protection compared with SD (rVE ranging from 10% to 19%), with no significant differences between HD, ADJ, and RIV. With enhanced vaccines combined, rVE versus SD was 18% (95% CI 3 to 32) from randomized and 11% (95% CI 8 to 14) from observational evidence. Meta-regression of observational studies suggested that those requiring laboratory confirmation of influenza reported greater benefit of enhanced vaccines.

CONCLUSIONS

HD, ADJ, and RIV provided stronger protection than SD against influenza hospitalizations among older adults. No differences in benefit were observed in comparisons of enhanced influenza vaccines with one another.

Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices - United States, 2024-25 Influenza Season

This report updates the 2023-24 recommendations of the Advisory Committee on Immunization Practices (ACIP) concerning the use of seasonal influenza vaccines in the United States (MMWR Recomm Rep 2022;72[No. RR-2]:1-24). Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. Trivalent inactivated influenza vaccines (IIV3s), trivalent recombinant influenza vaccine (RIV3), and trivalent live attenuated influenza vaccine (LAIV3) are expected to be available. All persons should receive an age-appropriate influenza vaccine (i.e., one approved for their age), with the exception that solid organ transplant recipients aged 18 through 64 years who are receiving immunosuppressive medication regimens may receive either high-dose inactivated influenza vaccine (HD-IIV3) or adjuvanted inactivated influenza vaccine (aIIV3) as acceptable options (without a preference over other age-appropriate IIV3s or RIV3). Except for vaccination for adults aged ≥65 years, ACIP makes no preferential recommendation for a specific vaccine when more than one licensed and recommended vaccine is available. ACIP recommends that adults aged ≥65 years preferentially receive any one of the following higher dose or adjuvanted influenza vaccines: trivalent high-dose inactivated influenza vaccine (HD-IIV3), trivalent recombinant influenza vaccine (RIV3), or trivalent adjuvanted inactivated influenza vaccine (aIIV3). If none of these three vaccines is available at an opportunity for vaccine administration, then any other age-appropriate influenza vaccine should be used.Primary updates to this report include the following two topics: the composition of 2024-25 U.S. seasonal influenza vaccines and updated recommendations for vaccination of adult solid organ transplant recipients. First, following a period of no confirmed detections of wild-type influenza B/Yamagata lineage viruses in global surveillance since March 2020, 2024-25 U.S. influenza vaccines will not include an influenza B/Yamagata component. All influenza vaccines available in the United States during the 2024-25 season will be trivalent vaccines containing hemagglutinin derived from 1) an influenza A/Victoria/4897/2022 (H1N1)pdm09-like virus (for egg-based vaccines) or an influenza A/Wisconsin/67/2022 (H1N1)pdm09-like virus (for cell culture-based and recombinant vaccines); 2) an influenza A/Thailand/8/2022 (H3N2)-like virus (for egg-based vaccines) or an influenza A/Massachusetts/18/2022 (H3N2)-like virus (for cell culture-based and recombinant vaccines); and 3) an influenza B/Austria/1359417/2021 (Victoria lineage)-like virus. Second, recommendations for vaccination of adult solid organ transplant recipients have been updated to include HD-IIV3 and aIIV3 as acceptable options for solid organ transplant recipients aged 18 through 64 years who are receiving immunosuppressive medication regimens (without a preference over other age-appropriate IIV3s or RIV3).This report focuses on recommendations for the use of vaccines for the prevention and control of seasonal influenza during the 2024-25 influenza season in the United States. A brief summary of the recommendations and a link to the most recent Background Document containing additional information are available at https://www.cdc.gov/acip-recs/hcp/vaccine-specific/flu.html?CDC_AAref_Val=https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/flu.html. These recommendations apply to U.S.-licensed influenza vaccines. Updates and other information are available from CDC's influenza website (https://www.cdc.gov/flu). Vaccination and health care providers should check this site periodically for additional information.

The challenges and benefits of public health in smart cities from a 4 M perspective

Introduction

With the acceleration of urbanization, public health issues have become increasingly prominent in smart city construction, especially in the face of sudden public health crises. A deep research method for public health management based on a 4M perspective (human, machine, materials, methods) is proposed to effectively address these challenges. Methods: The method involves studying the impact of human factors such as population age, gender, and occupation on public health from a human perspective. It incorporates a machine perspective by constructing a public health prediction model using deep neural networks. Additionally, it analyzes resource allocation and process optimization in public health management from the materials and methods perspectives.

Results

The experiments demonstrate that the public health prediction model based on deep neural networks achieved a prediction accuracy of 98.6% and a recall rate of 97.5% on the test dataset. In terms of resource allocation and process optimization, reasonable adjustments and optimizations increased the coverage of public health services by 20% and decreased the response time to public health events by 30%.

Discussion

This research method has significant benefits for addressing the challenges of public health in smart cities. It can improve the efficiency and effectiveness of public health services, helping smart cities respond more quickly and accurately to potential large-scale public health events in the future. This approach holds important theoretical and practical significance.

Contributions of cost-effectiveness analyses (CEA) to influenza vaccination policy for older adults in Europe

This review describes the importance of economic evaluations and real-world evidence (RWE) for the assessment of enhanced influenza vaccines for older adults in Europe. Individuals ≥65 years of age are at increased risk of severe influenza outcomes and many countries in Europe recommend enhanced vaccines for this population to mitigate immunosenescence. Some National Immunization Technical Advisory Groups (NITAGs) may preferentially recommend a specific enhanced vaccine, necessitating comparative economic evaluation and estimation of relative vaccine effectiveness between enhanced vaccine options in the absence of direct head-to-head efficacy data. Distinct approaches to economic modeling and cost-effectiveness analysis (CEA) guide national vaccination policies in Europe, including how underlying data, such as RWE, are used in these models. RWE is an important evidence source for input into CEA models based on disease factors (e.g., antigenic shift and seasonal variation) and practical factors (e.g., limitations of performing multiple randomized clinical trials to capture seasonal variation; the need to obtain relevant patient-oriented, real-world endpoints, such as hospitalizations). CEA is considered crucial to vaccine assessment among certain countries in Europe, but further harmonization of economic evaluations, including the use of RWE, across NITAGs in Europe may be of benefit, alongside standardized approaches for vaccine appraisal. In the future, more countries may use RWE as an input in CEA models to support NITAG recommendations for enhanced influenza vaccines in older populations, especially considering the value of RWE for the assessment of influenza epidemiology and vaccine effectiveness as stated by the World Health Organization, and the availability of a broad RWE base for certain enhanced vaccines.

Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices — United States, 2023–24 Influenza Season

This report updates the 2022–23 recommendations of the Advisory Committee on Immunization Practices (ACIP) concerning the use of seasonal influenza vaccines in the United States (

MMWR Recomm Rep 2022;71[No. RR-1]:1–28). Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. All seasonal influenza vaccines expected to be available in the United States for the 2023–24 season are quadrivalent, containing hemagglutinin (HA) derived from one influenza A(H1N1)pdm09 virus, one influenza A(H3N2) virus, one influenza B/Victoria lineage virus, and one influenza B/Yamagata lineage virus. Inactivated influenza vaccines (IIV4s), recombinant influenza vaccine (RIV4), and live attenuated influenza vaccine (LAIV4) are expected to be available.

For most persons who need only 1 dose of influenza vaccine for the season, vaccination should ideally be offered during September or October. However, vaccination should continue after October and throughout the season as long as influenza viruses are circulating and unexpired vaccine is available. Influenza vaccines might be available as early as July or August, but for most adults (particularly adults aged ≥65 years) and for pregnant persons in the first or second trimester, vaccination during July and August should be avoided unless there is concern that vaccination later in the season might not be possible. Certain children aged 6 months through 8 years need 2 doses; these children should receive the first dose as soon as possible after vaccine is available, including during July and August. Vaccination during July and August can be considered for children of any age who need only 1 dose for the season and for pregnant persons who are in the third trimester during these months if vaccine is available

ACIP recommends that all persons aged ≥6 months who do not have contraindications receive a licensed and age-appropriate seasonal influenza vaccine. With the exception of vaccination for adults aged ≥65 years, ACIP makes no preferential recommendation for a specific vaccine when more than one licensed, recommended, and age-appropriate vaccine is available. ACIP recommends that adults aged ≥65 years preferentially receive any one of the following higher dose or adjuvanted influenza vaccines: quadrivalent high-dose inactivated influenza vaccine (HD-IIV4), quadrivalent recombinant influenza vaccine (RIV4), or quadrivalent adjuvanted inactivated influenza vaccine (aIIV4). If none of these three vaccines is available at an opportunity for vaccine administration, then any other age-appropriate influenza vaccine should be used

Primary updates to this report include the following two topics: 1) the composition of 2023–24 U.S. seasonal influenza vaccines and 2) updated recommendations regarding influenza vaccination of persons with egg allergy. First, the composition of 2023–24 U.S. influenza vaccines includes an update to the influenza A(H1N1)pdm09 component. U.S.-licensed influenza vaccines will contain HA derived from 1) an influenza A/Victoria/4897/2022 (H1N1)pdm09-like virus (for egg-based vaccines) or an influenza A/Wisconsin/67/2022 (H1N1)pdm09-like virus (for cell culture-based and recombinant vaccines); 2) an influenza A/Darwin/9/2021 (H3N2)-like virus (for egg-based vaccines) or an influenza A/Darwin/6/2021 (H3N2)-like virus (for cell culture-based and recombinant vaccines); 3) an influenza B/Austria/1359417/2021 (Victoria lineage)-like virus; and 4) an influenza B/Phuket/3073/2013 (Yamagata lineage)-like virus. Second, ACIP recommends that all persons aged ≥6 months with egg allergy should receive influenza vaccine. Any influenza vaccine (egg based or nonegg based) that is otherwise appropriate for the recipient’s age and health status can be used. It is no longer recommended that persons who have had an allergic reaction to egg involving symptoms other than urticaria should be vaccinated in an inpatient or outpatient medical setting supervised by a health care provider who is able to recognize and manage severe allergic reactions if an egg-based vaccine is used. Egg allergy alone necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity of previous reaction to egg. All vaccines should be administered in settings in which personnel and equipment needed for rapid recognition and treatment of acute hypersensitivity reactions are available

This report focuses on recommendations for the use of vaccines for the prevention and control of seasonal influenza during the 2023–24 influenza season in the United States. A brief summary of the recommendations and a link to the most recent Background Document containing additional information are available at

https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/flu.html . These recommendations apply to U.S.-licensed influenza vaccines used according to Food and Drug Administration–licensed indications. Updates and other information are available from CDC’s influenza website ( https://www.cdc.gov/flu ). Vaccination and health care providers should check this site periodically for additional information.

[Expert opinion on strategies to improve vaccination coverage against seasonal influenza]

Seasonal flu continues to be a major public health concern, and the influenza vaccine remains the most effective preventive measure. In Spain, vaccination coverage data from previous seasons show vaccination rates well below official targets; however, these figures improved significantly after the COVID-19 pandemic. Given the importance of achieving and maintaining high vaccination rates in order to avoid the clinical and economic impact of influenza, our multidisciplinary group of experts on vaccines analyzed the impact of low vaccination rates in Spain and drafted a series of measures to boost influenza vaccination coverage, particularly among priority groups.

Prevention and Control of Seasonal Influenza with Vaccines: Recommendations of the Advisory Committee on Immunization Practices - United States, 2022-23 Influenza Season

This report updates the 2021–22 recommendations of the Advisory Committee on Immunization Practices (ACIP) concerning the use of seasonal influenza vaccines in the United States

(MMWR Recomm Rep 2021;70[No. RR-5]:1–24). Routine annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. For each recipient, a licensed and age-appropriate vaccine should be used.With the exception of vaccination for adults aged ≥65 years, ACIP makes no preferential recommendation for a specific vaccine when more than one licensed, recommended, and age-appropriate vaccine is available. All seasonal influenza vaccines expected to be available in the United States for the 2022–23 season are quadrivalent, containing hemagglutinin (HA) derived from one influenza A(H1N1)pdm09 virus, one influenza A(H3N2) virus, one influenza B/Victoria lineage virus, and one influenza B/Yamagata lineage virus. Inactivated influenza vaccines (IIV4s), recombinant influenza vaccine (RIV4), and live attenuated influenza vaccine (LAIV4) are expected to be available. Trivalent influenza vaccines are no longer available, but data that involve these vaccines are included for reference.

Influenza vaccines might be available as early as July or August, but for most persons who need only 1 dose of influenza vaccine for the season, vaccination should ideally be offered during September or October. However, vaccination should continue after October and throughout the season as long as influenza viruses are circulating and unexpired vaccine is available. For most adults (particularly adults aged ≥65 years) and for pregnant persons in the first or second trimester, vaccination during July and August should be avoided unless there is concern that vaccination later in the season might not be possible. Certain children aged 6 months through 8 years need 2 doses; these children should receive the first dose as soon as possible after vaccine is available, including during July and August. Vaccination during July and August can be considered for children of any age who need only 1 dose for the season and for pregnant persons who are in the third trimester if vaccine is available during those months

Updates described in this report reflect discussions during public meetings of ACIP that were held on October 20, 2021; January 12, 2022; February 23, 2022; and June 22, 2022. Primary updates to this report include the following three topics: 1) the composition of 2022–23 U.S. seasonal influenza vaccines; 2) updates to the description of influenza vaccines expected to be available for the 2022–23 season, including one influenza vaccine labeling change that occurred after the publication of the 2021–22 ACIP influenza recommendations; and 3) updates to the recommendations concerning vaccination of adults aged ≥65 years. First, the composition of 2022–23 U.S. influenza vaccines includes updates to the influenza A(H3N2) and influenza B/Victoria lineage components. U.S.-licensed influenza vaccines will contain HA derived from an influenza A/Victoria/2570/2019 (H1N1)pdm09-like virus (for egg-based vaccines) or an influenza A/Wisconsin/588/2019 (H1N1)pdm09-like virus (for cell culture–based or recombinant vaccines); an influenza A/Darwin/9/2021 (H3N2)-like virus (for egg-based vaccines) or an influenza A/Darwin/6/2021 (H3N2)-like virus (for cell culture–based or recombinant vaccines); an influenza B/Austria/1359417/2021 (Victoria lineage)-like virus; and an influenza B/Phuket/3073/2013 (Yamagata lineage)-like virus. Second, the approved age indication for the cell culture–based inactivated influenza vaccine, Flucelvax Quadrivalent (ccIIV4), was changed in October 2021 from ≥2 years to ≥6 months. Third, recommendations for vaccination of adults aged ≥65 years have been modified. ACIP recommends that adults aged ≥65 years preferentially receive any one of the following higher dose or adjuvanted influenza vaccines: quadrivalent high-dose inactivated influenza vaccine (HD-IIV4), quadrivalent recombinant influenza vaccine (RIV4), or quadrivalent adjuvanted inactivated influenza vaccine (aIIV4). If none of these three vaccines is available at an opportunity for vaccine administration, then any other age-appropriate influenza vaccine should be used

This report focuses on recommendations for the use of vaccines for the prevention and control of seasonal influenza during the 2022–23 influenza season in the United States. A brief summary of the recommendations and a link to the most recent Background Document containing additional information are available at

https://www.cdc.gov/vaccines/hcp/acip-recs/vacc-specific/flu.html. These recommendations apply to U.S.-licensed influenza vaccines used according to Food and Drug Administration–licensed indications. Updates and other information are available from CDC’s influenza website (https://www.cdc.gov/flu). Vaccination and health care providers should check this site periodically for additional information.

Systematic Review on the Cost-Effectiveness of Seasonal Influenza Vaccines in Older Adults

OBJECTIVES

Older adults are at high risk of influenza-related complications or hospitalization. The purpose of this systematic review is to assess the relative cost-effectiveness of all influenza vaccine options for older adults.

METHODS

This systematic review identified economic evaluation studies assessing the cost-effectiveness of influenza vaccines in adults ≥65 years of age from 5 literature databases. Two reviewers independently selected, extracted, and appraised relevant studies using the JBI Critical Appraisal Checklist for Economic Evaluations and Heyland's generalizability checklist. Costs were converted to 2019 Canadian dollars and adjusted for inflation and purchasing power parity.

RESULTS

A total of 27 studies were included. There were 18 comparisons of quadrivalent inactivated vaccine (QIV) versus trivalent inactivated vaccine (TIV): 5 showed QIV dominated TIV (ie, lower costs and higher health benefit), and 13 showed the results depended on willingness to pay (WTP). There were 9 comparisons of high-dose TIV (TIV-HD) versus TIV: 5 showed TIV-HD dominated TIV, and 4 showed the results depended on WTP. There were 8 comparisons of adjuvanted TIV (TIV-ADJ) versus TIV: 4 showed TIV-ADJ dominated TIV, and 4 showed the results depended on WTP. There were few pairwise comparisons among QIV, TIV-HD, and TIV-ADJ.

CONCLUSIONS

The evidence suggests QIV, TIV-HD, and TIV-ADJ are cost-effective against TIV for a WTP threshold of $50 000 per quality-adjusted life-year. Future studies should include new and existing vaccine options for broad age ranges and use more robust methodologies-such as real-world evaluations or modeling studies accounting for methodological, structural, and parameter uncertainty.

Estimation of Relative Vaccine Effectiveness in Influenza: A Systematic Review of Methodology

BACKGROUND

When new vaccine components or platforms are developed, they will typically need to demonstrate noninferiority or superiority over existing products, resulting in the assessment of relative vaccine effectiveness (rVE). This review aims to identify how rVE evaluation is being performed in studies of influenza to inform a more standardized approach.

METHODS

We conducted a systematic search on PubMed, Google Scholar, and Web of Science for studies reporting rVE comparing vaccine components, dose, or vaccination schedules. We screened titles, abstracts, full texts, and references to identify relevant articles. We extracted information on the study design, relative comparison made, and the definition and statistical approach used to estimate rVE in each study.

RESULTS

We identified 63 articles assessing rVE in influenza virus. Studies compared multiple vaccine components (n = 38), two or more doses of the same vaccine (n = 17), or vaccination timing or history (n = 9). One study compared a range of vaccine components and doses. Nearly two-thirds of all studies controlled for age, and nearly half for comorbidities, region, and sex. Assessment of 12 studies presenting both absolute and relative effect estimates suggested proportionality in the effects, resulting in implications for the interpretation of rVE effects.

CONCLUSIONS

Approaches to rVE evaluation in practice is highly varied, with improvements in reporting required in many cases. Extensive consideration of methodologic issues relating to rVE is needed, including the stability of estimates and the impact of confounding structure on the validity of rVE estimates.

The feasibility of pragmatic influenza vaccine randomized controlled real-world trials in Denmark and England

We estimated the frequency of non-specific influenza-associated clinical endpoints to inform the feasibility of pragmatic randomized controlled trials (RCT) assessing relative vaccine effectiveness (rVE). Hospitalization rates of respiratory, cardiovascular and diabetic events were estimated from Denmark and England's electronic databases and stratified by age, comorbidity and influenza vaccination status. We included a seasonal average of 4.5 million Danish and 7.2 million English individuals, 17 and 32% with comorbidities. Annually, approximately 1% of Danish and 0.5% of English individuals were hospitalized for selected events, ~50% of them respiratory. Hospitalization rates were 40-50-fold and 2-10-fold higher in those >50 years and with comorbidities, respectively. Our findings suggest that a pragmatic RCT using non-specific endpoints is feasible. However, for outcomes with rates <2.5%, it would require randomization of ~100,000 participants to have the power to detect a rVE difference of ~13%. Targeting selected groups (older adults, those with comorbidities) where frequency of events is high would improve trial efficiency.

Systematic review of the efficacy, effectiveness and safety of high-dose seasonal influenza vaccines for the prevention of laboratory-confirmed influenza in individuals ≥18 years of age

This review sought to assess the efficacy, effectiveness and safety of high-dose inactivated influenza vaccines (HD-IIV) for the prevention of laboratory-confirmed influenza in individuals aged 18 years or older. A systematic literature search was conducted in electronic databases and grey literature sources up to 7 February 2020. Randomised controlled trials (RCTs) and non-randomised studies of interventions (NRSIs) were included. The search returned 28,846 records, of which 36 studies were included. HD-IIV was shown to have higher relative vaccine efficacy in preventing influenza compared with standard-dose influenza vaccines (SD-IIV3) in older adults (Vaccine effectiveness (VE) = 24%, 95% CI 10-37, one RCT). One NRSI demonstrated significant effect for HD-IIV3 against influenza B (VE = 89%, 95% CI 47-100), but not for influenza A(H3N2) (VE = 22%, 95% CI -82 to 66) when compared with no vaccination in older adults. HD-IIV3 showed significant relative effect compared with SD-IIV3 for influenza-related hospitalisation (VE = 11.8%, 95% CI 6.4-17.0, two NRSIs), influenza- or pneumonia-related hospitalisation (VE = 13.7%, 95% CI 9.5-17.7, three NRSIs), influenza-related hospital encounters (VE = 13.1%, 95% CI 8.4-17.7, five NRSIs), and influenza-related office visits (VE = 3.5%, 95% CI 1.5-5.5, two NRSIs). For safety, HD-IIV were associated with significantly higher rates of local and systemic adverse events compared with SD-IIV (combined local reactions, pain at injection site, swelling, induration, headache, chills and malaise). From limited data, compared with SD-IIV, HD-IIV were found to be more effective in the prevention of laboratory-confirmed influenza, for a range of proxy outcome measures, and associated with more adverse events.

A Real-World Clinical and Economic Analysis of Cell-Derived Quadrivalent Influenza Vaccine Compared to Standard Egg-Derived Quadrivalent Influenza Vaccines During the 2019-2020 Influenza Season in the United States

Background

Cell-derived influenza vaccines are not subject to egg-adaptive mutations that have potential to decrease vaccine effectiveness. This retrospective analysis estimated the relative vaccine effectiveness (rVE) of cell-derived quadrivalent influenza vaccine (IIV4c) compared to standard egg-derived quadrivalent influenza vaccines (IIV4e) among recipients aged 4-64 years in the United States during the 2019-2020 influenza season.

Methods

The IQVIA PharMetrics Plus administrative claims database was utilized. Study outcomes were assessed postvaccination through the end of the study period (7 March 2020). Inverse probability of treatment weighting (IPTW) was implemented to adjust for covariate imbalance. Adjusted rVE against influenza-related hospitalizations/emergency room (ER) visits and other clinical outcomes was estimated through IPTW-weighted Poisson regression models for the IIV4c and IIV4e cohorts and for the subgroup with ≥1 high-risk condition. Sensitivity analyses modifying the outcome assessment period as well as a doubly-robust analysis were also conducted. IPTW-weighted generalized linear models were used to estimate predicted annualized all-cause costs.

Results

The final sample comprised 1 150 134 IIV4c and 3 924 819 IIV4e recipients following IPTW adjustment. IIV4c was more effective in preventing influenza-related hospitalizations/ER visits as well as respiratory-related hospitalizations/ER visits compared to IIV4e. IIV4c was also more effective for the high-risk subgroup and across the sensitivity analyses. IIV4c was also associated with significantly lower annualized all-cause total costs compared to IIV4e (-$467), driven by lower costs for outpatient medical services and inpatient hospitalizations.

Conclusions

IIV4c was significantly more effective in preventing influenza-related hospitalizations/ER visits compared to IIV4e and was associated with significantly lower all-cause costs.

Effectiveness of the quadrivalent high-dose influenza vaccine for prevention of cardiovascular and respiratory events in people aged 65 years and above: Rationale and design of a real-world pragmatic randomized clinical trial

BACKGROUND

Influenza has been an acknowledged cause of respiratory disease for decades. However, considerable related, and often unappreciated, disease burden stems from cardiovascular complications, exacerbations of underlying medical conditions and secondary respiratory complications, with the highest burden in the elderly. This novel study combines the gold standard method of a randomized controlled trial with real-world data collection through national registries, to assess the relative effectiveness of high-dose (QIV-HD) vs standard-dose quadrivalent influenza vaccine (QIV-SD) in preventing cardio-respiratory hospitalizations in a large cohort of adults aged ≥65 years.

METHODS AND RESULTS

This trial (NCT04137887) is a Phase III/IV, modified double-blinded, randomized, registry-based trial, conducted by the Finnish Institute for Health and Welfare (THL). Participants (n>120 000) are being enrolled over multiple influenza seasons and randomized (1:1) to receive QIV-HD or QIV-SD. Participant follow-up is based on data collection up to 11 months post-vaccination using Finnish national health registries. The primary objective is to demonstrate the relative superior effectiveness of QIV-HD over QIV-SD in preventing cardio-respiratory hospitalizations up to 6 months post-vaccination. Safety will be assessed using automated online tools throughout the study, with causality assessed using statistical and probabilistic methods; serious adverse reactions and adverse events of special interest will be investigated individually.

CONCLUSION

This large, real-world, randomized study will provide valuable insight into the contribution of influenza in causing severe cardio-respiratory events, and the role of vaccination with QIV-HD in reducing these outcomes compared to the current standard of care.

FUNDING

Sanofi Pasteur.

Safety and immunogenicity of high doses of quadrivalent influenza vaccine in children 6 months through <18 years of age: A randomized controlled phase II dose-finding trial

Quadrivalent high-dose inactivated influenza vaccine (Fluzone® High-Dose Quadrivalent, IIV4-HD) was licensed in the USA in 2019 for adults ≥ 65 years of age. This Phase II study examined safety and immunogenicity of 3 dose formulations of IIV4-HD in healthy children. In a randomized, modified double-blind, active-controlled trial in the USA and Canada, 661 children aged 6 months through < 18 years received 1 or 2 doses intramuscularly of standard-dose quadrivalent influenza vaccine (IIV4-SD; 15 µg HA/strain), IIV4-HD at 3 dose levels (30, 45, and 60 µg HA/strain), or adjuvanted trivalent influenza vaccine (aIIV3, 7.5 µg HA/strain). Rates of unsolicited AEs were similar irrespective of dose. No treatment-related serious adverse events or deaths were reported. Reactogenicity was slightly higher for IIV4-HD than IIV4-SD, although most solicited reactions were grade 1 or 2. Hemagglutination inhibition (HAI) and seroneutralization antibody titers were measured 28-35 days after each dose. Geometric mean HAI titers increased with increasing hemagglutinin dose, especially in children 6 months through < 3 years. For IIV4-HD 60 µg, in participants 6 months through < 18 years of age, the geometric mean titer ratio (95% confidence interval) versus IIV4-SD was 1.35 (0.94, 1.94) for A/H1N1, 2.51 (1.77, 3.55) for A/H3N2, 1.60 (1.17, 2.18) for B/Victoria, and 1.51 (1.13, 2.03) for B/Yamagata. The GMT ratio (95% confidence interval) for IIV4-HD 60 µg versus IIV4-SD was highest for participants 6 months through < 3 years of age: 4.24 (2.05, 8.76) for A/H1N1, 3.14 (1.53, 6.44) for A/H3N2, 2.04 (1.10, 3.77) for B/Victoria, and 1.92 (1.08, 3.41) for B/Yamagata; similarly, seroneutralization antibody GMT ratio was highest in these participants: 170 (84.6, 340) for A/H1N1, 7.13 (4.90, 10.4) for A/H3N2, 35.8 (22.1, 58.1) for B/Victoria, and 22.7 (14.7, 35.0) for B/Yamagata. This study showed that IIV4-HD (60 µg HA/strain) provides improved immunogenicity without affecting vaccine safety in children.

Introductory paper: High-dose influenza vaccine

Seasonal influenza has a significant impact on global public health each year, especially in older adults 65 years of age and above. This paper presents the evolution of high-dose influenza vaccine and the quantity as well as quality of evidence on this vaccine. Its introduces other peer-reviewed manuscripts included in this supplement covering the benefits high-dose influenza vaccine over ten consecutive influenza seasons. The development of the high-dose influenza vaccine represents an important step in the evolution of influenza vaccines, offering an advancement in prevention of influenza and a step in encouraging healthy aging in older adults. A video summary of the article can be accessed via the Supplementary data link at the end of this article.

Efficacy and effectiveness of high-dose influenza vaccine in older adults by circulating strain and antigenic match: An updated systematic review and meta-analysis

BACKGROUND

Influenza vaccine efficacy/effectiveness can vary from season to season due in part to the dominant circulating strains and antigenic matching. This study reviews the relative vaccine efficacy/effectiveness (rVE) of high-dose inactivated trivalent influenza vaccine (HD-IIV3) compared to standard-dose influenza vaccines (SD-IIV) in adults aged ≥ 65 years against influenza-associated outcomes. Additional sub-analyses of HD-IIV3 rVE were performed by the predominantly circulating influenza strain and the antigenic match or mismatch of the vaccine against the predominant circulating strains.

METHODS

An updated systematic review and meta-analysis was conducted for studies assessing the rVE of HD-IIV3 against probable/laboratory-confirmed influenza-like illness (ILI), hospital admissions, and death in adults aged ≥ 65 years. Results from individual seasons were extracted from the studies, and viral surveillance data were used to determine the dominant circulating strains and antigenic match for each season. Results were then stratified based on clinical outcomes and seasonal characteristics and meta-analyzed to estimate pooled rVEs of HD-IIV3.

RESULTS

15 publications were meta-analyzed after screening 1,293 studies, providing data on 10 consecutive influenza seasons and over 22 million individuals receiving HD-IIV3 in randomized and observational settings. Across all influenza seasons, HD-IIV3 demonstrated improved protection against ILI compared to SD-IIV (rVE = 15.9%, 95% CI: 4.1-26.3%). HD-IIV3 was also more effective at preventing hospital admissions from all-causes (rVE = 8.4%, 95% CI: 5.7-11.0%), as well as influenza (rVE = 11.7%, 95% CI: 7.0-16.1%), pneumonia (rVE = 27.3%, 95% CI: 15.3-37.6%), combined pneumonia/influenza (rVE = 13.4%, 95% CI: 7.3-19.2%) and cardiorespiratory events (rVE = 17.9%, 95% CI: 15.0-20.8%). Reductions in mortality due to pneumonia/influenza (rVE = 39.9%, 95% CI: 18.6-55.6%) and cardiorespiratory causes (rVE = 27.7%, 95% CI: 13.2-32.0%) were also observed. Similar pooled rVEs were observed in both matched and mismatched seasons and in seasons where A/H3N2 or A/H1N1 strains were predominantly circulating.

CONCLUSIONS

Evidence over 10 consecutive influenza seasons and in more than 34 million individuals aged ≥ 65 years suggests that HD-IIV3 is consistently more effective than SD-IIV at reducing influenza cases as well as influenza-associated clinical complications irrespective of circulating strain and antigenic match. A video summary of the article can be accessed via the Supplementary data link at the end of this article.

Cost-effectiveness of high dose versus adjuvanted trivalent influenza vaccines in England and Wales

AIMS

High dose trivalent influenza vaccine (HD TIV) and adjuvant TIV (aTIV) have been developed specifically for adults aged 65 and older (65+) who are at high risk of life-threatening complications. However, there is a scarcity of evidence comparing the clinical and cost-effectiveness of HD TIV and aTIV. The aim of this study was to determine the cost-effectiveness of HD TIV versus aTIV in the England and Wales 65+ population.

METHODS

A cost-utility analysis was conducted using a decision tree with two influenza related outcomes: Laboratory confirmed cases that could result in GP consultation, and hospitalizations that may result in premature mortality. Due to a lack of comparative evidence, the effectiveness of HD TIV versus aTIV was calculated indirectly, based on relative effectiveness estimates for each vaccine versus a common comparator, standard dose (SD) TIV. The primary analysis included hospitalizations explicitly due to influenza/pneumonia. Cost-effectiveness was established for three scenarios applying differing relative effectiveness estimates for aTIV versus SD TIV. Uncertainty was analysed in one-way deterministic sensitivity analyses. A secondary analysis included hospitalizations due to any respiratory illness.

RESULTS

The minimum population impact of vaccination with HD TIV rather than aTIV was 13,092 fewer influenza cases, 1,109 fewer influenza related deaths, 4,673 fewer hospitalizations, and 3,245 fewer GP appointments. HD TIV was cost-effective versus aTIV for all three effectiveness scenarios, with incremental cost-effectiveness ratios (ICER) equal to £1,932, £4,181, and £8,767 per quality adjusted life year. Results were consistent across the secondary analysis and deterministic sensitivity analyses.

LIMITATIONS

The analysis was limited by a lack of robust and consistent effectiveness data for aTIV.

CONCLUSION

HD TIV is cost-effective versus aTIV in people aged 65+ in England and Wales. Use of HD TIV over aTIV could increase clinical benefits and reduce the public health and economic burden of influenza.

High-dose influenza vaccination and mortality among predominantly male, white, senior veterans, United States, 2012/13 to 2014/15

Introduction

It is unclear whether high-dose influenza vaccine (HD) is more effective at reducing mortality among seniors.

Aim

This study aimed to evaluate the relative vaccine effectiveness (rVE) of HD.

Methods

We linked electronic medical record databases in the Veterans Health Administration (VHA) and Medicare administrative files to examine the rVE of HD vs standard-dose influenza vaccines (SD) in preventing influenza/pneumonia-associated and cardiorespiratory mortality among VHA-enrolled veterans 65 years or older during the 2012/13, 2013/14 and 2014/15 influenza seasons. A multivariable Cox proportional hazards model was performed on matched recipients of HD vs SD, based on vaccination time, location, age, sex, ethnicity and VHA priority level.

Results

Among 569,552 person-seasons of observation, 207,574 (36%) were HD recipients and 361,978 (64%) were SD recipients, predominantly male (99%) and white (82%). Pooling findings from all three seasons, the adjusted rVE estimate of HD vs SD during the high influenza periods was 42% (95% confidence interval (CI): 24–59) against influenza/pneumonia-associated mortality and 27% (95% CI: 23–32) against cardiorespiratory mortality. Residual confounding was evident in both early and late influenza periods despite matching and multivariable adjustment. Excluding individuals with high 1-year predicted mortality at baseline reduced the residual confounding and yielded rVE of 36% (95% CI: 10–62) and 25% (95% CI: 12–38) against influenza/pneumonia-associated and cardiorespiratory mortality, respectively. These were confirmed by results from two-stage residual inclusion estimations.

Discussion

The HD was associated with a lower risk of influenza/pneumonia-associated and cardiorespiratory death in men during the high influenza period.

Comparative effectiveness of influenza vaccines among U.S. Medicare beneficiaries ages 65 years and older during the 2019-20 season

BACKGROUND

Around 50,000 influenza-associated deaths occur annually in the U.S., overwhelmingly among individuals ages >65 years. Although vaccination is the primary prevention tool, investigations have shown low vaccine effectiveness (VE) in recent years, particularly among the elderly. We analyzed the relative VE (RVE) of all influenza vaccines among Medicare beneficiaries ages >65 years to prevent influenza hospital encounters during the 2019-20 season.

METHODS

Retrospective cohort study using Poisson regression and inverse probability of treatment weighting (IPTW). Exposures included egg-based high-dose trivalent (HD-IIV3), egg-based adjuvanted trivalent (aIIV3), egg-based standard dose (SD) quadrivalent (IIV4), cell-based SD quadrivalent (cIIV4), and recombinant quadrivalent (RIV4) influenza vaccines.

RESULTS

We studied 12.7 million vaccinated beneficiaries. Following IPTW, cohorts were well balanced for all covariates and health-seeking behavior indicators. In the adjusted analysis, RIV4 (RVE 13.3%, 95% CI 7.4%, 18.9%), aIIV3 (RVE 8.2%, 95% CI 4.2%, 12.0%), and HD-IIV3 (RVE 6.8%, 95% CI 3.3%, 10.1%) were significantly more effective in preventing hospital encounters than the reference egg-based SD IIV4, while cIIV4 was not significantly more effective than IIV4 (RVE 2.8%, 95% CI -2.8%, 8.2%). Our results were consistent across all analyses.

CONCLUSIONS

In this influenza B-Victoria and A(H1N1)-dominated season, RIV4 was moderately more effective than other vaccines, while the HD-IIV3 and aIIV3 were more effective than the IIV4 vaccines, highlighting the contributions of antigen amount and adjuvant use to VE. Egg adaptation likely did not substantially affect our RVE evaluation. Our findings, specific to the 2019-20 season, should be evaluated in other studies using virological case confirmation.

[High-dose trivalent influenza vaccine. Efficacy and effectiveness]

Seasonal influenza is a major public health problem, particularly in older people. Influenza vaccine is the most effective way to prevent influenza virus infection and its complications, but due to immunosenescence, older people do not respond efficiently to immunization. In 2009, a high-dose trivalent influenza vaccine (IIV3-HD), containing four times more antigen than the standard-dose vaccine, was approved in the United States for the immunization of people aged 65 years and over. Numerous clinical trials, carried out at different seasons and using different methodologies, have shown that the IIV3-HD vaccine is, as well as safe, more immunogenic and more effective than the standard-dose vaccine in preventing influenza virus infection and its complications in older people. This paper reviews the available evidence on the efficacy and effectiveness of the IIV3-HD influenza vaccine in the elderly, with information from randomized clinical trials, as well as observational studies of real-world clinical practice and in systematic reviews/meta-analyses.

High-Dose Inactivated Influenza Vaccine Quadrivalent for Older Adults

OBJECTIVE

To review the efficacy and safety of the high-dose inactivated influenza vaccine quadrivalent (HD-IIV4) in the prevention of influenza in older adults.

DATA SOURCES

A literature search was performed using PubMed and Google Scholar with the search terms high-dose, influenza vaccine, and quadrivalent. Other resources included the Centers for Disease Control and Prevention (CDC), the prescribing information, and the manufacturer's website.

STUDY SELECTION AND DATA EXTRACTION

All relevant English-language articles of studies assessing the efficacy and safety of HD-IIV4 were included.

DATA SYNTHESIS

HD-IIV4 is licensed by the Food and Drug Administration for the prevention of influenza in adults aged 65 years and older. The safety and immunogenicity of HD-IIV4 was demonstrated in a phase 3 trial, and the efficacy of the trivalent formulation (HD-IIV3) was demonstrated in a phase 3b-4 trial. HD-IIV4 carries a warning regarding the occurrence of Guillain-Barré syndrome. Adverse reactions, including injection-site pain and myalgia, were reported more frequently with HD-IIV4 than with HD-IIV3.

RELEVANCE TO PATIENT CARE AND CLINICAL PRACTICE

Although the CDC recommends any age-appropriate influenza vaccine for adults aged 65 years and older, HD-IIV4 was associated with improved immunogenicity against the added B strain and HD-IIV3 provided better protection against influenza than the standard-dose vaccine. Other influenza vaccines have weaker evidence of efficacy in older adults. Therefore, HDIIV4 should be recommended as the vaccine of choice in adults aged 65 years and older.

CONCLUSION

HD-IIV4 has proven immunogenic, safe, and effective in preventing influenza in older adults and should be recommended as the vaccine of choice in this patient population.

Comparing the impact of high-dose versus standard dose influenza vaccines on hospitalization cost for cardiovascular and respiratory diseases: Economic assessment in the US Veteran population during 5 respiratory seasons using an instrumental variable method

OBJECTIVES

Cost savings associated with high-dose (HD) as compared to standard-dose (SD) influenza vaccination in the United States (US) Veteran's Health Administration (VHA) population have been attributed to better protection against hospitalization for cardiac and respiratory diseases. The relative contribution of each of these disease categories to the reported savings remains to be explored.

METHODS

During a recently completed study of HD versus SD vaccine effectiveness (conducted in the VHA over five respiratory seasons from 2010/11 through 2014/15), we collected cost data for all healthcare services provided at both VHA and Medicare-funded facilities. In that analysis, we compared the costs of vaccination and hospital care for patients admitted with either cardiovascular or respiratory disease. Treatment selection bias and other confounding factors were adjusted using an instrumental variable (IV) method. In this brief report we use the same study cohort and methods to stratify the results by patients admitted for cardiovascular disease (CVD) and those admitted for respiratory disease.

RESULTS

We analyzed 3.5 million SD and 0.16 million HD person-seasons. The IV-adjusted rVEs were 14% (7-20%) against hospitalizations for CVD and 15% (5-25%) against respiratory hospitalizations. Net cost savings per HD recipient were $138 ($66-$200) for CVD related hospitalizations and $62 ($10-$107) for respiratory disease related hospitalizations.

CONCLUSIONS

In the US VHA population, the reduction in hospitalizations for CVD over five respiratory seasons contributed twice the cost savings (per HD recipient) of the reduction in hospitalizations for respiratory disease.

Immunogenicity and safety of high-dose quadrivalent influenza vaccine in Japanese adults ≥65 years of age: a randomized controlled clinical trial

A trivalent high-dose inactivated influenza vaccine has been licensed in healthy adults ≥65 years of age and provides better protection against influenza infection and related complications than trivalent standard-dose vaccine. This phase I/II clinical trial (NCT03233217), conducted at two sites in Japan, examined the safety and immunogenicity of a quadrivalent formulation of the high-dose inactivated influenza vaccine (IIV4-HD). Healthy adults ≥65 years of age were randomized to receive IIV4-HD by intramuscular injection (n = 60), IIV4-HD by subcutaneous injection (n = 60), or a quadrivalent standard-dose inactivated influenza vaccine (IIV4-SD) by subcutaneous injection (n = 55). Irrespective of administration route, post-vaccination (day 28–35) hemagglutination inhibition geometric mean titers and seroconversion rates were higher for IIV4-HD than for IIV4-SD. Hemagglutination inhibition geometric mean titers and seroconversion rates were also higher for intramuscular than subcutaneous administration of IIV4-HD. Solicited reactions were more common in participants who received IIV4-HD administered subcutaneously than in those who received IIV4-HD administered intramuscularly or IIV4-SD administered subcutaneously. Unsolicited adverse events were similar between the vaccine groups, and no safety signals were detected. This study showed that IIV4-HD administered by either intramuscular or subcutaneous injection was well tolerated and highly immunogenic in healthy Japanese adults ≥65 years of age. Although this study was descriptive, the results add to the evidence that high-dose inactivated influenza vaccines are more immunogenic than standard-dose vaccines in this age group and that intramuscular administration provides greater immunogenicity and lower reactogenicity than subcutaneous administration.

Comparative effectiveness of high dose versus adjuvanted influenza vaccine: A retrospective cohort study

BACKGROUND

Adults 65 years and older (seniors) experience more complications following influenza infection than younger adults. We estimated the relative vaccine effectiveness (rVE) of a trivalent high dose (HD-IIV3) versus an adjuvanted trivalent influenza vaccine (aIIV3) in seniors for respiratory-related hospitalizations.

METHODS

We conducted a retrospective cohort study using claims data from Optum's Clinformatics® Data Mart to compare outcome rates between seniors who received HD-IIV3 versus aIIV3 during the 2016/17 and 2017/18, predominantly A/H3N2 respiratory seasons. Rates were adjusted for demographic characteristics, comorbid conditions, previous influenza vaccination, and geography. We used the previous event rate ratio (PERR) approach to address bias by time-fixed unmeasured confounders.

RESULTS

We identified 842,282 HD-IIV3 and 34,157 aIIV3 recipients for the 2016/17 season and 1,058,638 HD-IIV3 and 189,636 aIIV3 recipients for the 2017/18 season. The pooled rVE of HD-IIV3 versus aIIV3 for respiratory-related hospitalizations over both seasons was 12% (95% confidence interval: 3.3%-20%); 13% (-6.4% to 32%) for the 2016/17 season and 12% (2.1%-21%) for the 2017/18 season.

CONCLUSIONS

Pooled over two predominantly A/H3N2 respiratory seasons, HD-IIV3 was associated with fewer respiratory hospital admissions than aIIV3 in senior members of large national managed health care company in the U.S.

Safety and immunogenicity of high-dose quadrivalent influenza vaccine in adults ≥65 years of age: A phase 3 randomized clinical trial

BACKGROUND

A high-dose, split-virion inactivated trivalent influenza vaccine (IIV3-HD; Fluzone® High-Dose, Sanofi Pasteur) is available for adults ≥65 years of age. This study examined the safety and immunogenicity of a quadrivalent high-dose split-virion inactivated influenza vaccine (IIV4-HD).

METHODS

This was a randomized, modified double-blind, active-controlled, multi-center trial in healthy adults ≥65 years of age. Subjects were randomized in a 4:1:1 ratio to receive a single intramuscular injection of IIV4-HD, the licensed IIV3-HD, or an IIV3-HD containing the alternate B-lineage strain. Hemagglutination inhibition (HAI), seroneutralisation, and anti-neuraminidase antibody titers were measured at baseline and day 28. Solicited reactions were collected for up to 7 days, unsolicited adverse events up to 28 days, and serious adverse events up to 180 days. The primary immunogenicity objective was to demonstrate that IIV4-HD induces HAI geometric mean titers (GMTs) and seroconversion rates that are non-inferior to those induced by IIV3-HD. Secondary objectives were to describe the safety of IIV4-HD and IIV3-HD and to demonstrate that IIV4-HD induces HAI GMTs and seroconversion rates that are superior to those induced by IIV3-HD not containing the same B-lineage strain.

RESULTS

The study included 2670 adults ≥65 years of age. For all four strains, HAI GMTs and seroconversion rates induced by IIV4-HD were non-inferior to those induced by IIV3-HDs containing the same strains. For both B strains, HAI GMTs and seroconversion rates induced by IIV4-HD were superior to those induced by IIV3-HD not containing the same B-lineage strain. Seroneutralisation and anti-neuraminidase antibody responses, measured in a subset of subjects, were similar. No new safety concerns were identified, and the safety profiles of IIV4-HD and IIV3-HD were similar.

CONCLUSIONS

Adding a second B strain in IIV4-HD resulted in improved immunogenicity against the added strain without compromising the immunogenicity of the other strains or the vaccine's tolerability.

CLINICAL TRIAL REGISTRATION

NCT03282240.